There are several methods for evaluating existing lighting environments as well as methods for predicting the illuminating effectiveness of unrealized lighting environments. Most such techniques currently utilize the concept of Equivalent Sphere Illumination (ESI).
One indirect method involves a number of measurements and calculations and employs the use of a standard visual task such as a hand written pencil lettering on a white paper background. The illumination on the task provided by the lighting environment is measured. The luminance of the visual task is measured from a position which substantially corresponds to the position of the observer in the act of reading the visual task by using a nonpolarizing luminance meter.
Then, using a visual task photometer, the actual contrast of the visual task is measured from the normal eye position as defined above. The visual task is then moved to a position where it is illuminated by sphere illumination and the visual task photometer is used to measure the contrast under these conditions. Then the two measured contrasts are compared. This comparison usually entails a series of calculations and reference to a number of standardized, tabulated data to correct for any difference between the luminances under the two lighting conditions. The final comparison is usually expressed in terms of Equivalent Sphere Illumination.
This method is outlined in detail in RQQ Report #4, Illuminating Engineering, Volume 65, August 1970, Pages 504-510.
To date the only way to directly measure ESI for a particular lighting environment is with a prior art meter which involves subjective visual judgment by a trained operator. This method, as outlined in U.S. Pat. Nos. 3,912,399 and 4,055,383, uses a visual observation in order to compare directly the visibility of a standard task rendered by the lighting environment in question with the same or identical reference task illuminated by a sphere illumination. The visibility of the reference task positioned in the lighting environment is reduced to threshhold by degrading the contrast between the background and the task itself. This is done by utilizing a graduated neutral density filter. Substantially simultaneously with this degradation, the background luminace is supplemented by an external light source. In the preferred embodiment this supplemental luminance is reflected off the front of the graduated neutral density filter. This maintains the background luminance at the proper level so that the degradation has the net effect of decreasing the contrast only, not the overall task luminance. The visual task, or an identical visual task sample, is then viewed under sphere illumination. The image of the task under sphere illumination remains under an identical degraded condition. The level of sphere illumination is adjusted to the point of threshhold visibility. An illumination reading is taken of the sphere illumination in order to determine its magnitude. The illumination level determined by this reading is that sphere illumination which is equivalent (in terms of its contrast rendering ability) to the illumination provided by the lighting environment--hence is the Equivalent Sphere Illumination or ESI.
This last method, while simplifying considerably the number of steps involved in determining ESI, requires a skilled operator in order to obtain consistent results. That is, the person making the visual comparisons and determining under what conditions of contrast degradation the visibility of the task is effectively extinguished needs considerable experience with the machine. Without such experience, brightness rather than visibility tends to be the determining criterion. Clearly the necessity for a skilled operator has limited the dissemination of the method and apparatus disclosed in these two patents. Also, since the operator must be a skilled operator, it cannot be said that the operator was a "typical" observer such as would be utilizing the lighting environment being evaluated. Hence, even if skilled operators are able to obtain consistent results, these results would not necessarily correspond to "typical observer" data.
Another system unlike the above instrument disclosed in the above cited patents, utilizes a pair of generally transparent cylinders placed over an ordinary footcandle meter. These cylinders include partially opaque portions which transmit light with varying degrees in order to simulate the effectiveness of light, arriving from various angular zones from the lighting environment, in producing Equivalent Sphere Illumination footcandles. Details of this instrument are set forth in the Journal of the Illuminating Engineering Society, Vol. 7, No. 3, April 1978, P. 183.
Contrasting with the above systems for evaluating an existing lighting environment, there also exist systems for predicting the Equivalent Sphere Illumination as well as other parameters such as contrast rendition factor (CRF) of yet to be realized lighting environments. One such system constructs a mathematical model of the lighting environment, taking into cosideration the effect of the room size and shape, surface reflectivity of the room itself, the characteristics of the lighting system in question as well as the parameters associated with the task observer, such as location, line of sight, viewing angle, the nature of the visual task, as well as body shadow effects. The first portion of this predictive technique requires a sophisticated computational method which discretizes various luminous and reflecting portions of the room, reducing the illumination therefrom to discretized and quantitized candle power figures. Once so described, the luminance of the task and background (and hence ESI) can be predicted by applying gonio-reflectance factors to these illumination calculations. The gonio-reflectance factors to these illumination calculations. The gonio-reflectance factors describe the response of standard visual tasks as a function of light received from various directions or zones of the environment surrounding the task. These gonio-reflectance factors have been determined through experimental techniques. This data is available in tabulated form and is expressed in luminance factors as a function of degrees of zenith and degrees in azimuth of the particular ray or rays of light emanating from a zone of the lighting environment and illuminating the task location for particular standardized observer positions. This method is detailed in RQQ Report #5; Journal of the Illuminating Engineering Society, January 1973, pp. 149-166.